Such pumps, e.g. molecular pumps or turbomolecular pumps which can maintain pressures lying between 1 mb and 10.sup.-10 mb, rotate at very high speeds, and it is possible for the rotorsupporting bearings, and also for the gaps in the driving electric motor and in the dynamic seal separating the active portion of the pump from the portion containing the motor and the bearings all to be affected by the suction of very small physical particles which lead, in the end, to said members being damaged.
In order to avoid sucking in such particles, it is known that the pump suction inlet can be fitted with one or more filters constituted by very fine mesh metal gauze having a mesh of about 10 microns to 20 microns at most.
These filters do indeed stop such particles, however they have very low conductance. In this range of pressures, flow conditions are molecular or viscous, and as a result the molecular conductance of a filter pore is very low since this conductance is proportional to the cube of the pore diameter and inversely proportional to the pore length.
Such poor filter conductance considerably reduces the pumping rate of the pump. In addition to poor filter conductance, present filters clog rapidly, thereby quickly reducing pumping rates.